The present invention relates to an electroluminescent wafer and a method of fabricating the same and, particularly, to an electroluminescent wafer having an epitaxial layer of a compound semiconductor material consisting of elements belonging to groups IIIb and Vb of periodical table (hereinafter, referred to as III-V semiconductor) and a method of fabricating the same electroluminescent wafer.
A thin layer of III-V semiconductor provided on a substrate by an epitaxial growth has been used in fabricating semiconductor devices such as a light emitting diode (hereinafter referred to as LED). It has been known, however, that III-V semiconductor material generally have a high index of refraction and, therefore, a substantial portion of light emitted from the p-n junction of an LED provided with an epitaxial layer of III-V semiconductor may be fully reflected at an interface provided by a surface of the layer. In this case, when a substrate on which the epitaxial layer is deposited is of monocrystalline material exhibiting a poor light transparency, most of the light emitted may be absorbed within the substrate of the LED.
For example, the index of refraction of gallium arsenide phosphide (GaAs.sub.(1-x) Px, x=0.4) is about 3.6 and the total reflection angle thereof is about 16.degree.. Therefore, in an LED comprises a substrate of gallium arsenide having poor light transparency and an epitaxial layer of gallium arsenide phosphide deposited thereon, the percentage of the total amount of light which is emitted from the p-n junction thereof and which can be used, is very small and generally 3 to 4% of the total light emitted.
Further, if the substrate is of a III-V semiconductor different from that forming the epitaxial layer or of a semiconductor consisting of an element such as germanium (Ge) belonging to group IV of the periodic table, the epitaxial wafer obtained may possibly be warped because of differences in properties of material such as crystallographic lattice constant between the substrate and the epitaxial layer.
In order to provide ohmic electrode on the substrate side of the wafer, it is usual to polish the wafer to facilitate the provision of electrodes. With a curved epitaxial wafer, there is a possibility that the inner surface of the epitaxial layer will be locally exposed by the polishing.
It is generally difficult to form electrodes directly on a polished surface of a low carrier density epitaxial layer suitable to form a p-n junction. This disadvantage may become serious when a curved wafer is used to produce LEDs or when a substrate having poor light transparency, which will be removed, after the epitaxial layer is formed, to increase the amount of usable light, is used to form the wafer.
That is, in either of the above cases, the yield of LEDs may be significantly decreased due to the difficulty of forming ohmic electrodes on the low carrier density layer.